I had to consult esther drill, the creator of the famous zit popping game: blemish remover in order to offer you a complete answer. here is what esther says:

you ask some good questions.

let me start with the difference between blackheads and whiteheads and how they get formed --> the pores of your skin contain sebaceous (pronounced se-bay-shis) glands that make sebum (pronounced see-bum), which is an oil for your hair and skin. usually, the right amount of sebum is made and everything is fine. but sometimes a pore gets clogged up with too much sebum and dead skin cells.

this is likely to happen when you're a teenageer because of those crazy hormones that come with puberty. as your body changes and matures, the hormones stimulate the glands to make more oil and you end up with overactive oil-producing glands.

if a pore gets clogged up, but stays open, the top surface can get dark and then you have a blackhead. if a pore gets clogged up and closes and begins bulging from the skin, then you have a whitehead. if bacteria gets into a pore, it can infect the oil gland, which produces redness and that disgusting whitish yellowish pus -- this is the classic pimple. you will often see whiteheads on top of these pimples.

ok, even though i wrote a whole article about the joys and necessities of popping zits, i feel compelled at this point to be responsible and repeat the official dermatologist-approved line about popping zits --> don't touch, squeeze, pick at, or pop your zits. squeezing or popping pimples can force the oil and bacteria into the deeper skin layers, which can cause cysts. you can make pimples worse by poking at them or opening them up. picking at pimples can leave small but permanent scars on your face. as you can see, there are a lot of good reasons to leave your pimples alone.

now, if you insist on disregarding this official dermatological advice...well then, read on.

If you pop a blackhead effectively, the hard clear junk that comes out is the clogged oil and dead skin. if you try to pop a pimple with a whitehead, you will most often squeeze out some of the gross pus--that's the gooey stuff you saw. if you squeeze too hard, you may break skin and draw blood. that's probably where the red stuff came from.